Carbon and its compounds play an important role in our world, from life itself to the stability of our climate. A lot of research effort has been invested in the global carbon cycle in the biosphere, atmosphere, hydrosphere and shallow sub-surface, in particular how this cycle is responding to the burning of fossil fuels and the release of carbon dioxide into the atmosphere.
Much less is known about the carbon stored deep under the surface of the Earth. We don’t know how much of it is there, as the estimates vary by a factor of 20. Other important and unanswered questions concern the mobility of ‘deep carbon’ and its exchange with the carbon cycle at the surface, ie carbon fluxes; the limits of sub-surface life; and the fundamental properties of carbon materials under the high pressures present in the deep sub-surface.
Deep Carbon Observatory
In 2007, Jesse Ausubel, programme director of the Alfred P. Sloan Foundation, an independent grant-making institution in New York, US, encouraged Robert Hazen from the Carnegie Institution, in Washington, DC, to organise a workshop charged with examining the unanswered questions about carbon in the Earth. The workshop, The deep carbon cycle, held the following year with 115 people from 12 countries, led to the launch in 2009 of the Deep Carbon Observatory (DCO) – a 10-year interdisciplinary, international research programme funded by the Sloan Foundation. By 2011, the DCO had established four directorates, covering:
- reservoirs and fluxes (RF) of carbon in the planet;
- deep energy (DE), or the implications for energy;
- deep life (DL), exploring the deep biosphere; and
- extreme physics and chemistry (EPC) of carbon materials.
There are now close to 1000 scientists from 60 countries involved in these areas of research, and already they are beginning to provide some interesting answers to some of the fundamental questions underpinning the project.
Arguably the most elementary question that the DCO is aiming to address is how much carbon there is in the Earth. Knowing these data will allow planetary scientists to draw conclusions about other terrestrial (rocky) planets in our Solar System and beyond.
The total planetary carbon that we know about adds up to 0.07% (w/w) of the Earth, says Jesse Ausubel, but literature estimates range up to 1.5%. The carbon content of carbonaceous chondrites, a class of meteorites believed to be representative of the material from which the inner planets formed, is 3.2%.
Carbon below the surface of the Earth can occur in more than a dozen different chemical forms, from diamonds to hydrocarbons, and for all of these the distribution and abundance are unknown. Similarly, the carbon fluxes remain poorly quantified. One can estimate, for instance, how much carbon gets pulled down into the mantle at subduction zones, where oceanic plates slide underneath the continents, but the fraction of that carbon returning to the surface in, for example, volcanic gases, may be anywhere between 2% and 75%.
The RF directorate, chaired by Erik Hauri from the Department of Terrestrial Magnetism of the Carnegie Institution, aims to make significant improvements in measuring the carbon fluxes, based on measurements in situ and on satellite observations. This work will require instrumentation for both laboratory experiments and field observations, with at least an order of magnitude better sensitivity, resolution, and accuracy than current equipment.
‘Quantifying volcanic degassing is a major challenge of our directorate, and it all comes down to determining the pre-eruptive carbon content of magmas from volcanoes around the world; the 80,000km system of submarine mid-ocean ridge volcanoes and 44,000km of subduction zone volcanoes,’ says Hauri. ‘Of the roughly 550 historically active volcanoes on Earth, only five are monitored for carbon flux and so this represents a huge uncertainty. Then there are natural diamonds, formed at great depth beneath the continents. Their antiquity and toughness make them the most robust recorders of the deep carbon cycle as far back as three billion years ago.’
Carbon fluxes on Earth are extremely important because methane or carbon dioxide released from geological sources may add to the climate worries we already have. Beyond our own planet, however, insights into the deep carbon cycle may also help us to understand the chemical make-up of other rocky planets. For instance, when carbon compounds are discovered on Mars, it will be easier to verify if they are of biological or abiotic (non-living, eg chemical reactions at high temperature and pressure), origin if we understand these issues better. Indeed, research done last July by Andrew Steele and colleagues from the Geophysical Laboratory at Carnegie reported the discovery of reduced carbon macromolecules in 10 out of 11 Martian meteorites they investigated (Science, 2012, 337, 212). These results suggest that terrestrial planets like Earth and Mars have complex abiotic carbon compounds, so we can’t use these as a proxy when looking for life.
Historically, much geological research has been motivated by the quest for natural resources, and most prominently by our continuing, though dangerous, dependence on fossil fuels. The textbook wisdom has it that oil and natural gas are the fossil remains of living organisms.
However, there are also alternative hypotheses suggesting that to some extent, abiotic methane may be released from deep within the Earth. Given the right set of high pressures and temperatures, this methane could be converted into higher hydrocarbons without any biological involvement, as researchers have shown in laboratory experiments in 2009 (A. Kolesnikov et al, Nature Geoscience, 2009, 2, 566).
The DE directorate, chaired by David Cole from Ohio State University, US, aims to investigate natural hydrocarbon sources for signs of biological or abiotic origins, for example in isotopic signatures. This work will require detailed experiments to measure carbon isotopes under controlled chemical conditions that simulate the deep Earth, backed up by laboratory experiments.
‘The answer to this grand challenge may reside in our ability to measure the rarest of carbon/deuterium/hydrogen isotope ratio combinations in the most dominant of the hydrocarbon gases: methane,’ Cole explains. ‘The DCO is sponsoring the development of revolutionary types of instrumentation at UCLA and MIT to measure the relative abundance of doubly-isotope substituted methane, also known as ‘clumped isotopes’. These results will provide a unique and critical constraint on the origin(s) of deep methane by allowing evaluation of the temperature at which the methane was produced or internally equilibrated for its 13C–D bond abundance.’
Interest in abiotic hydrocarbons on Earth has been revived by the recent discoveries of hydrocarbons elsewhere in the Solar System. As the Cassini-Huygens mission confirmed, Saturn’s moon Titan has a complete ‘hydrological’ system running on hydrocarbons, with ethane clouds, rain, rivers, and lakes. Space probes orbiting Mars have also detected methane on this planet.
While many of the open questions concerning the ‘deep carbon cycle’ revolve around abiotic carbon compounds, there is also the distinct possibility that life in the sub-surface of the Earth extends much deeper than we cuurently know. A few key discoveries from the past decade have indicated that simple ecosystems using chemical energy sources can thrive more than a mile below ground. But are such rock-eating microbial communities the exception or the rule in the deep sub-surface? And where exactly is the boundary of the biosphere?
The DL directorate, chaired by Isabelle Daniel from the Université Claude Bernard at Lyon, France, aims to find, sequence and cultivate microbial samples from deep sub-surface habitats around the world. High hydrostatic pressures of up to 1100 bar, as found in the deepest trenches of the ocean, require specific adaptations, but don’t rule out life.
Earlier research from the Geophysical Laboratory indicated that some microbial species can survive extreme pressures up to 14,000 bar (A. Sharma et al, Science, 2002, 295, 1514). This has been confirmed in more recent work that points to microbial viability at pressures above 20,000 bar (D. Vanlint et al, mBio, 2011, 2, e00130-10).
Depending on how abundant and how deep-reaching the sub-surface ecosystems are, these may well be making a significant contribution to the carbon fluxes and the generation and turnover of hydrocarbons.
Interest in the deeper reaches of the biosphere, again, has implications beyond the confines of Earth, as similar, deep geological habitats probably exist on Mars as well and may have offered refuge to any remnants of earlier Martian life.
Finally, the EPC directorate, chaired by Giulia Galli, at the University of California, Davis, US, is looking into the chemistry and physics under the extremes of pressure and temperature found in the deeper layers of the Earth. Using experiments, observations, and modelling, researchers will investigate the thermodynamics and kinetic behaviour of the phases believed to carry carbon.
Familiar materials may adopt very different structural properties under high pressures in the Earth. For instance, theoretical predictions by Artem Oganov and colleagues at the State University of New York at Stony Brook, US, suggest that magnesium and calcium carbonate adopt a structure with carbon surrounded by four oxygen atoms, rather than the usual two or three found at lower pressures. Under these extreme conditions, carbonate minerals are more akin to silicate mineral structures at atmospheric pressure (Earth Planet. Sci. Lett., 2008, 273, 38). These structural changes are similar to the conversion of graphite to diamond at high pressures.
Stewart McWilliams from the Carnegie Institution and colleagues from Berkeley and Livermore Universities in the US have investigated the behaviour of magnesium oxide under deep mantle conditions. They found that the transitions between the phases of this material display large latent heat release, which is relevant to an understanding of the formation and evolution of rocky planets, especially of super-Earths – ie those planets outside our Solar System, which have a larger mass than Earth but a smaller mass than either Neptune or Uranus. In addition, the researchers found that the material can conduct electricity and thus contribute to a planet’s ‘dynamo’ producing the magnetic field (Science, 2012, 338, 1330).
A group of scientists, including some from McWilliams’ team, has now used the National Ignition Facility (NIF), located at the Lawrence Livermore National Laboratory in California, US, to measure the compression of the carbon directly, starting as diamond, to record pressures above 50 megabars. ‘These experiments are opening up entirely new vistas for exploring the nature of matter and materials under extreme conditions. Such conditions have never been been created in the laboratory before. It is especially significant that these first experiments are being done on carbon, whose physics and chemistry are so important,’ said Russell Hemley, director of the Geophysical Laboratory and co-principal investigator of the DCO. Related experiments are under way on hydrogen as well.
The researchers setting up the DCO have set themselves ambitious targets for the 10-year lifetime of their project, aiming to improve our understanding of crucial aspects of those parts of our planet that we cannot easily reach. ‘Success within this decade requires not only new samples, but also new ways of sampling and instruments variously more sensitive, smaller, larger, more robust, and less susceptible to contamination,’ comments Jesse Ausubel.
Eventual benefits could be reaped in the fields of climate change and monitoring of geological activity. ‘Its legacies could include a comprehensive global monitoring network for outgassing from volcanoes and other major sources as well as vastly enhanced Earth data infrastructure,’ says Ausubel.
Beyond that, all parts of the project resonate with the recent excitement in the wider field of astrobiology, fuelled by successful Solar System missions and the discovery of many extrasolar planets with an increasing number of Earth-like planets among them. Earth science is no longer just about Earth, it is also about Earth as an example of the many millions of planets in the Universe. It is also about how these planets originate, evolve and possibly develop to support life.
Peering inside the navel of our own planet we can thus begin to address some of the biggest questions in the Universe.
Michael Gross is a science writer based in Oxford, UK.